Imagine that UVs are clothing on a person. A particular person that doesn’t want to be naked. But a person can wear different types of clothes, right? So in order to understand what fits, you need to know sizes and certain parameters of that person: height, shoe size, shoulder width, chest size, etc. You need to measure it. Same with UVs. You need to unwrap the surface of an object and tell the program what that particular texture will look like.
As for painting on models: it’s just a way to make something fast. If you want to make a really tight and precise texture — unwrapping and texture painting is the way to succeed.
